Circuit to control DC MOTOR 100Wat with reverse

Thread Starter

marce002

Joined Jan 20, 2019
14
Hi

I want to power a RC car DC powerful RS-540SH Motor with a servo that rotates a potenciometer to control speed (removed the original mechanic speed control), so i have made a H bridge circuit like attached , it works fine with reverse and forward but lot of noise and consuption at middle position when motor is not turning at all... also not reaching top speed, like volts are 6v instead of 8.4v ,.. what can I do to get rid of that noise? also 2.5A when iddle!! which is bad
In case you think this approach is incorrect, what would be the best circuit to control the cars speed with reverse and forward for this application, maybe removing the servo too and use directly the pwm signal from the receiver, or in case to keep this circuit, how can I improve it, maybe mosfets or add caps or any idea THANKS IN ADVANCE


circuito.png
 
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Thread Starter

marce002

Joined Jan 20, 2019
14
from a youtube video


worked indeed but with all the details that i have mentioned... maybe you should know that currently the car has a mechanical speed control with two ceramical 5watt resistors to disipate the heat when the car runs low speed (by design), and has only 2 discrete speeds,,,, it is from 1987!!!, so the circuit might be a step ahead, but of course I need to improve if you can help!
 

schmitt trigger

Joined Jul 12, 2010
843
The circuit is a very simplistic approach with so many flaws that I can’t list them all.
One of them, if I understand the circuit correctly is that during the PWM’s “high” time it powers the motor in one direction, during the “low” time it powers it in the opposite direction.
In other words, it controls the motor speed by accelerating and braking alternatively.

At “idle”, it is braking and accelerating simultaneously, thus the large current draw.
All in all an extremely poor design. Small wonder that the video has the comments disabled, it would have been mercilessly criticized.
I will search something better for your application.

EDIT: More accurately than braking, actually powering the motor in reverse.
 
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Thread Starter

marce002

Joined Jan 20, 2019
14
thanks a lot schmitt, that one is 3.4A a bit underpowered... I really apreciate your help if you think of another design. It is not posible to buy anything pre made since it is going to be a documented project, I think if not I would not be here posting and probably buy a 30 or 50A brushed ESC (not the case) thanks again and if you recall something else to build will apreciate it quite much, sorry my English...(from bsas AR)
 

Martin_R

Joined Aug 28, 2019
137
Their will be a problem with the RC cars radio pwm value. It varies as a pulse between 1mS-2mS at a 50hz repetition rate, it's a standard that radio control uses. The standard is 1.5mS centers a servo with 1mS sending it in one direction and 2mS in the opposite direction.
 

Thread Starter

marce002

Joined Jan 20, 2019
14
Thanks Martin and schmitt , i coud not find a suitable easy and cheap solution so far.... nothing
I have never imagined that a 100w 12vol rc car brushed rev/forward ESC (or motor dc driver or whatever) controlled by a PWN RC standar would be so difficult.... I m now trying to mod a micro servo , and have its 4 micro fets Y1 and Y2 removed, and replace them with bigger ones ( details here ), but still asking myself will it worth ..... clearly not happy with the results so far...
 

LowQCab

Joined Nov 6, 2012
3,937
Trying to modify a Servo-Circuit-Board to make it control a much larger Motor
has got to be the biggest waste of Time and Money that I have ever witnessed.

But who knows ?,
maybe it will do something "entertaining" for a few minutes.
.
.
.
 

Martin_R

Joined Aug 28, 2019
137
I don't think that will be a feasible solution. A servo has a position potentiometer which feeds back to the control to say it's got into correct position. It then cuts off the drive pulses to the servo motor. It does not drive the motor once it's in position. I'm not quite sure how your mod will work . I can only imagine the motor with hunt back and forth in its stationary position!
 

Thread Starter

marce002

Joined Jan 20, 2019
14
ok I understand, let s then think about what would be the simplest solution ...maybe, it will be something like this:

just a pot 10k and a IRF Z44N , but how would you mod this to make reverse from the receiver?
1699556156866.png

1699556144593.png
 

LowQCab

Joined Nov 6, 2012
3,937
A Double-Pole-Double-Throw DPDT Relay will also be required for reversing the Motor,
and,
an extremely large Heat-Sink will also be required,
otherwise the FET will only last a few seconds before smoking,
and,
from full-stop to full-Motor-speed will be in an extremely narrow range of rotation of the Pot,
probably less than ~5-degrees of rotation.

Virtually all practical Motor-Controllers use PWM Switching for
all of the above reasons, plus many other various advantages.
.
.
.
 

Martin_R

Joined Aug 28, 2019
137
I would use a motor speed controller as in post #5, it's the cheapest and power efficient way to do it. Then as lowQ said use a relay to swop direction. Could use a second speed controller to turn the relay on and off to change direction, effectively 1mS pulse would be relay off and 2mS pulse relay on, changing direction.
You would need 2 channels though.
Actually, you said this is an RC car, but have you got a radio set for it, as the original circuit posted as not for RC.
 

Thread Starter

marce002

Joined Jan 20, 2019
14
ok thanks! perfect answers... it is very clear, i will opt for 2realys option with a big heat sink just for now, a few attempts revealed a bit more than 5 degrees pot rotation though...(i have used 50kohm) thanks a lot BYE//
 
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